I understand the , and the in the previous post, are some constant, but what constant?

What am I supposed to do with all that? Where did all that come from? T.T

it works for any and all integers. it is just there to show that the answer repeats itself and infinite number of times. so it works if we add 1 pi, or 2 pi or 3pi or ....

how you deal with the constants depends on the original question. if the question asked for , then you should only choose those integers that cause the solution to fall in that interval, and you are generally expected to list all solutions in this interval, since it is not usually too many.